Increased integration density requires semiconductor devices having increasingly miniaturized features. As gate lengths are reduced, problems such as short channel effects are encountered. For example, "punch through" arises when the drain voltage reaches a sufficiently large value, and the depletion layer associated with the drain spreads across the substrate and reaches the source, thereby enabling the charge carriers in the drain to punch through to the source and increasing leakage current significantly. Large amount of "punch through" charge carriers may deteriorate a transistor's function completely even with sufficient gate voltage to turn off the channel, resulting in complete loss of its controllability as a switch. In addition, "hot carrier injection" arises when device dimensions are reduced but the supply voltage is maintained, thereby increasing the electric field generated in the silicon substrate. Such an increased electric field enables electrons in the channel region to gain sufficient energy to be injected onto the gate oxide, resulting in device degradation.
There exists a need for semiconductor methodology and devices exhibiting improved short channel characteristics with reduced punch through and hot carrier injection.